1. Field of the Invention
The present invention relates to methods and apparatus for separating mixtures of articles of different densities and more particularly to such methods and apparatus as are applicable to the sorting of articles such as, for example, agricultural or other products having small density differences into several density groups by utilizing a flotation stream.
2. Background Art
The use of density variation as a means of separating mixtures of articles is widespread. In agriculture, the separation and sorting of produce on this basis is accomplished using both wet and dry methods.
Wet methods use a liquid as a medium with which to separate denser articles, which sink in the given liquid, from the lighter ones that will float thereupon. Dry methods of sorting employ a form of pneumatic separation based on a combination of differing densities and differing aerodynamic properties associated with the components to be sorted.
In one type of dry method, a gas, such as air, is forced upwardly through a moving bed of the mixture to be separated. This gas flow through the interstices of the particles of the mixture tends to disengage the particles from each other, permitting the gas flow to support at least some of the weight thereof. As a result, the bed of the mixture resembles a liquid of high viscosity, and the particles of the mixture are freed to a degree to migrate within the bed under the influence of physical forces such as gravity that might tend to induce separation among the constituent components.
The separation that occurs when a mixture to be separated is itself fluidized is not one that results exclusively due to differing density among the components of the mixture. Instead, the aerodynamic properties of the particles of the mixture also have a substantial impact upon the rate and quality of the separation that results. The upward flow of gas through the mixture will tend to draw with it the less compact particles of the mixture, regardless of their density.
In one type of device the fluidization of such a mixture may be effected as it passes down an inclined trough. At the discharge end of the trough the mixture of the materials will become somewhat stratified according to the combined density and aerodynamic property of the component particles. Nevertheless, such a device has several inherent drawbacks which render it less than optimally desirable in relation to the broad range of circumstances in which agriculture separators of the dry variety are nevertheless desirable.
First, separators which pneumatically fluidize the actual mixture to be separated have limited separation effectiveness. While the upper and lower layers of the stratified mixture discharged from the end of the separator trough may be relatively pure, the layers intermediate thereto continue to comprise a mixture of particles of both densities. This problem is ameliorated to some degree by horizontally narrowing the separation between the vertical walls of the trough in the vicinity of its discharge end. This has the effect of increasing the depth of the flow at that point, affording more vertical distance between the separated top and bottom layers of the mixture Still, at some point between these two extreme layers, the two materials of differing densities remain substantially intermixed in an interfaced layer. This fact precludes the achievement of optimal separation effectiveness.
A second, more profound drawback of separation methods in which the mixture to be separated is itself pneumatically fluidized arises from the fact that fluidization of the mixture is not practical if the particles of the mixture have diameters greater than approximately three or four millimeters. Thus, such methods are effective only in separating small products, such as grain cereal. They cannot be used to separate or sort large produce.
Toward that end, resort has been made to a second type of dry method which is based on the use of fluidized beds which are constituted of a material other than the mixture to be separated. For the purpose of separating mixtures of larger solid bodies of differing densities, a fluidized bed created from such a fluidization medium behaves in a manner analogous to a liquid, but without wetting the articles of the mixture it is used to separate. Pieces of solid material less dense than the apparent density of the fluidized bed will act as a "float fraction" which will float on the surface of the bed. Pieces of solid material which are more dense than the apparent density of the fluidized bed will, on the other hand, act as a "sink fraction" of the mixture which will sink to the bottom of the bed.
For separation to occur, the apparent density of the fluidized bed must be intermediate the densities of the float and sink fractions of the mixture. Additionally, the particle size of the fluidization medium must be smaller by several orders of magnitude than the size of the bodies contained in the mixture that is to be separated.
The use of a fluidization medium other than the mixture to be separated advantageously reduces the influence on the process of other separation factors, such as aerodynamic characteristics, and reduces the process to one in which separation is accomplished substantially on the basis of differing density only. In addition, the presence of a layer of fluidization medium intermediate the float fraction of the mixture on top of the fluidized bed and the sink fraction of the mixture at the bottom thereof permits a clean separation of the float and sink fractions. This is accomplished by separating the upper portion of the fluidized bed with the float fraction entrained therein from the lower portion thereof having the sink fraction entrained therein. Thereafter the two components are cleaned independently to remove any fluidization medium, and close to one hundred percent separation effectiveness between the float and sink fractions of the mixture can be achieved.
While this type of dry method works well for many applications, it still has some limitations. For example, most of the available methods, except a few wet methods, are aimed at separating products with large differences in density (such as clods and stones from potatoes, or plastic particles from copper particles, etc.). Meanwhile, a large variety of sorting applications for various types of products exists wherein the products to be sorted have only small differences in density. Mixtures of such products are commonly found, for example, in agriculture.
Most agricultural products such as fruits and vegetables do not have uniform quality and they do not uniformly mature. Postharvest quality sorting is thus required to supply reliable and uniform quality in the market place.
Some of the used techniques for quality sorting are specific to the kinds of produce for which they were developed. In addition, there are no viable methods for sorting numerous other products.
Density may be the most direct and consistent index of maturity and other quality changes. When quality changes are not manifest in external changes, such as differences in size, weight, color, etc., quality sorting with current technology is ineffective. Density may be the sole criterion to permit opportunities for quality sorting in such circumstances. However, quality sorting based on density differences has found limited success in commercial applications for several reasons.
Density changes due to quality transformations in agricultural commodities usually are very small (in the range of 0.02-0.04 g/cm.sup.3). Effectively detecting and sorting products having these small differences appears to be possible only in a highly controlled density sorting process. Current density sorting techniques which use liquids such as brine solutions or solutions of alcohol in water to sort sink and float fractions require very close control of the density of such solutions so as to maintain the density intermediate that of the sink and float fractions. This is difficult, particularly as such solutions tend to become contaminated with foreign materials, which affects the solution's density. Thus, frequent changing of the solution may be required, as is preconditioning and post washing operations to reduce contamination and also to remove such liquids from the produce. These operations often deteriorate product quality and storageability. Furthermore, such liquids are expensive, and they may present fire and social hazards when used in large quantities. Some commodities such as peas and blueberries need preliminary prewetting to remove air bubbles. Others, such as peanuts, walnuts, and pecans generally can't be processed in liquids because the absorption of the liquids adversely change mealiness properties. Furthermore, sorting frequently requires grading into three or more categories, which in turn may require several liquid changes.
On the other hand, dry methods of the type noted above are generally limited to sorting mixtures of products wherein there are relatively large density differences between the float and sink fractions. When differences in densities of the products to be sorted are small, such as in the range of differences on the order of 0.02 g/cm.sup.3, the density of the fluidized bed, which as noted must be intermediate the sink and float fractions, should differ by only 0.01 g cm.sup.3 from the densities of the products. Maintaining a fluidized bed within such parameters is technically difficult to achieve. Accordingly, what is needed is a method and apparatus for sorting articles with small density differences which can be implemented utilizing either wet or dry fluidized bed techniques and which eliminates many of the above-noted difficulties. Such an apparatus and method are described and claimed herein.